1. Technical Field
The present invention relates to an image sensor, more particularly to a CMOS image sensor and a controlling method thereof.
2. Description of the Related Art
An image sensor refers to a semiconductor device converting an optical image to an electric signal. The image sensor includes a charge coupled device (CCD) and a complementary metal-oxide-semiconductor (CMOS) image sensor. The CCD is composed of an integrated circuit containing metal-oxide-semiconductor (MOS) capacitors disposed adjacent to each other, and each capacitor can transfer its electric charge to a neighboring capacitor. The CMOS image sensor is formed by using a CMOS technology, which employs a control circuit and a signal processing circuit as peripheral circuits, is composed of as many MOS transistors as the number of pixels, and detects an output from each MOS transistor one by one.
FIG. 1 is a sectional view of a pair of pixels having different sizes in a conventional image sensor. As shown in FIG. 1, a conventional CCD expands a dynamic range by using a large pixel 110 and a small pixel 120.
It takes the small pixel 120 four times as much time as the large pixel 110 to be saturated with inputted light of the same intensity. Accordingly, with a pair of pixels generating different outputs, the dynamic range can be expanded. Here, the dynamic range refers to a range within which a maximum output of the image sensor can change, and the wider the dynamic range becomes, the more realistically an object can be represented.
FIGS. 2a and 2b illustrate images generated by applying different output voltages, and FIG. 2c illustrates a composite image of the images in FIGS. 2a and 2b. A composite image can be created by combining a short exposure image and a long exposure image.
The brightness of the composite image can be controlled by regulating the ratio of the output voltage required for producing the short exposure image to the output voltage required for producing the long exposure image. Therefore, the brightness of the composite image can be set between the brightness of the short exposure image and that of the long exposure image.
However, a minute difference in manufacturing process creates a fixed pattern noise associated with an offset voltage in the image sensor. In order to compensate such a fixed pattern noise, the image sensor reads a reset voltage signal and a data voltage signal from each pixel in a pixel array, and then outputs the difference between the two voltage signals. This method is called a correlated double sampling (CDS).
Here, the CCD has disadvantages in that the CCD has a complex operating process, consumes a large volume of power, requires a number of mask process steps, and is manufactured as a multi-chip device because a signal processing circuit can not be implemented in the CCD chip. In order to overcome these disadvantages, a CMOS image sensor, which uses sub-micron CMOS manufacturing technology, has been developed. The CMOS image sensor detects signals by a switching method from a photo diode and a MOS transistor formed in a unit pixel to create an image. Since the CMOS image sensor is manufactured by CMOS manufacturing technology, it consumes less-power and requires fewer masks than the CCD, and can be integrated in one chip with other signal processing circuits. Therefore, the CMOS image sensor is emerging as an alternative to the CCD.
Also, in the case of pairing a large pixel with a small pixel, photo diodes should also be formed to have different sizes, complicating the manufacturing process.
And, in the case of pairing a large pixel with a small pixel, each pixel needs its own transistor, increasing the overall size of the image sensor.
Moreover, in order to obtain a wide dynamic range, the conventional CCD image sensor stores data on the short exposure image and the long exposure image in separate frame memories and then conducts an operation, thereby requiring a large volume of memory.